This invention relates to magnetic steel plates exhibiting satisfactory magnetic properties, including magnetic plates which can be used for magnetic shielding from leakage magnetic flux. This invention also relates to a method of manufacturing such steel plates.
In recent years, many high-technology devices which utilize a strong magnetic field have been developed. One typical apparatus which uses very strong magnetic fields is a magnetic resonance imaging apparatus (hereunder referred to as an "MRI apparatus").
During the operation of an MRI apparatus there is a large amount of leakage magnetic flux. As the leakage magnetic flux can adversely affect electrical equipment outside the MRI apparatus, it is important to shield the surroundings from the leakage magnetic flux. There are two methods of providing magnetic shielding. One is to cover the MRI apparatus itself with magnetic shielding members, and the other is to surround the room where the MRI apparatus is installed with magnetic shielding members. In either method, the shielding members are usually steel plates with a high degree of magnetic permeability. Such steel plates are called magnetic leakage-shielding steel plates, and are also used as covering members and structural members of large-scale equipment for scientific research such as cyclotrons in order to carry out magnetic shielding.
Therefore, such magnetic steel plates must have satisfactory mechanical properties, and there is a strong need for a material which has not only good mechanical properties but also good magnetic properties such as permeability and magnetic flux density.
Soft magnetic steel plates have been used as magnetic flux-shielding members. The most-widely used one is a thin plate for use in transformers. The steel plates defined in JIS C 2504 are thin plates with a thickness of 0.6-4.5 mm. JIS C 2503 defines steel bars having a diameter of 1.0-16 mm.
There are also cases in which a steel plate such as S10C steel which is defined in JIS G 4051 as a mechanical structural carbon steel plate is employed as a magnetic steel.
In addition, Japanese Published Unexamined Patent Application No. 96749/1985, Japanese Published Examined Patent Application No. 45442/1988, and Japanese Published Examined Patent Application No. 45443/1988 disclose a thick steel plate for use in direct current magnetization, which contains a rather large amount of sol. Al, e.g. 0.005-1.00% of sol. Al and as little of Si as possible. This steel plate is made from a low carbon steel which has been deoxidized with Al.
However, the magnetic properties of these conventional magnetic steel plates are not adequate for the plates to shield the leakage flux such as is experienced in MRI apparatuses.
(i) Soft magnetic bars and plates such as defined in JIS C 2503 and 2504 are intended to be used as small-sized parts. They are not intended to be used as structural members and their mechanical properties are poor. Therefore, if such a magnetic plate is to be applied to an MRI apparatus, it is necessary to laminate about 10 steel sheets in order to obtain adequate rigidity. This manufacturing method is impractical because of high manufacturing costs and poor quality of the laminated product.
(ii) The carbon steels for mechanical and structural use which are defined in JIS G 4051 have a maximum permeability (.mu..sub.max) of 1800 or smaller. This is because magnetic properties are not regarded as important for such materials.
The magnetic steel plate disclosed in Japanese Published Unexamined Patent Application No. 96749/1985 has a maximum permeability (.mu..sub.max) which extends over a wide range of 12850 to 4260. The permeability of that steel is not adequate for the steel to be used as a magnetic steel plate for shielding the leakage magnetic flux from an MRI apparatus.
According to the methods disclosed in Japanese Published Examined Patent Application No. 45442/1988 and No. 45443/1988, it is possible to increase the maximum permeability (.mu..sub.max) of a steel plate to 2000-5000. However, this level of permeability is inadequate for the steel plate to be used in an MRI apparatus.
Thus, it is not possible to obtain a satisfactory magnetic steel plate for use as a magnetic shielding member in devices such as MRI apparatuses.